diff --git a/local_build/local_build.sh b/local_build/local_build.sh
index c5122036c..2355555db 100755
--- a/local_build/local_build.sh
+++ b/local_build/local_build.sh
@@ -20,7 +20,7 @@
 #
 # Usage: local_build.sh.
 #
-# This script generates only local_blockly_compressed.js.  You may modify it as
+# This script generates only local_blockly_compressed.js. You may modify it as
 # needed to build other files.
 #
 # The compressed file is a concatenation of all of Blockly's core files, run
@@ -33,6 +33,16 @@
 # - Add a good error message if multiple versions of the closure compiler were
 #   found.
 
+# Note that the root folder MUST be named 'local_build'.
+
+EXPECTED_PWD='local_build'
+if [[ ${PWD##*/} != $EXPECTED_PWD ]]; then
+  echo ""
+  echo "  WARNING   This script should be run from the blockly/local_build folder."
+  echo ""
+  exit 1
+fi
+
 # Find the Closure Compiler.
 if [ -f "$(npm root)/google-closure-compiler/compiler.jar" ]; then
   COMPILER="$(npm root)/google-closure-compiler/compiler.jar"